Pingwen Zhang

Analysis of the heterogeneous multiscale method for elliptic homogenization problems

The heterogeneous multiscale method (HMM) is applied to various parabolic problems with multiscale coefficients. These problems can be either linear or nonlinear. Optimal estimates are proved for the error between the HMM solution and the homogenized solution.

Moving Mesh Finite Element Methods for the Incompressible Navier--Stokes Equations

This work presents the first effort in designing a moving mesh algorithm to solve the incompressible Navier--Stokes equations in the primitive variables formulation. The main difficulty in developing this moving mesh scheme is how to keep it divergence-free for the velocity field at each time level. The proposed numerical scheme extends a recent moving grid method based on harmonic mapping [R. Li, T. Tang, and P. W. Zhang, J. Comput. Phys., 170 (2001), pp. 562--588], which decouples the PDE solver and the mesh-moving algorithm. This approach requires interpolating the solution on the newly generated mesh. Designing a divergence-free-preserving interpolation algorithm is the first goal of this work. Selecting suitable monitor functions is important and is found challenging for the incompressible flow simulations, which is the second goal of this study. The performance of the moving mesh scheme is tested on the standard periodic double shear layer problem. No spurious vorticity patterns appear when even fairly coarse grids are used.

Analysis of the heterogeneous multiscale method for parabolic homogenization problems

The heterogeneous multiscale method (HMM) is applied to various parabolic problems with multiscale coefficients. These problems can be either linear or nonlinear. Optimal estimates are proved for the error between the HMM solution and the homogenized solution.

Discontinuous galerkin time-domain method for GPR simulation in dispersive media

This paper presents a newly developed high-order discontinuous Galerkin time-domain (DGTD) method for solving Maxwells equations in linear dispersive media with UPML boundary treatment. A unified formulation is derived for linear dispersive media of Debye type and the artificial material in the UPML regions with the help of auxiliary differential equations. The DGTD employs finite-element-type meshes, and uses piecewise high-order polynomials for spatial discretization and Runge-Kutta method for time integrations. Arbitrary high-order accuracy can be obtained for scattering of various objects in dispersive media. After validating the numerical convergence of the DGTD method together with the second-order Yees scheme, we apply this new method to the ground-penetrating radar for the detection of buried objects in a lossy half space.

An adaptive mesh redistribution method for nonlinear Hamilton--Jacobi equations in two-and three-dimensions

This paper presents an adaptive mesh redistribution (AMR) method for solving the nonlinear Hamilton-Jacobi equations and level-set equations in two- and three-dimensions. Our approach includes two key ingredients: a nonconservative second-order interpolation on the updated adaptive grids, and a class of monitor functions (or indicators) suitable for the Hamilton-Jacobi problems. The proposed adaptive mesh methods transform a uniform mesh in the logical domain to cluster grid points at the regions of the physical domain where the solution or its derivative is singular or nearly singular. Moreover, the formal second-order rate of convergence is preserved for the proposed AMR methods. Extensive numerical experiments are performed to demonstrate the efficiency and robustness of the proposed adaptive mesh algorithm.

Discontinuous galerkin time domain (DGTD) methods for the study of 2-D waveguide-coupled microring resonators

This paper presents the study of coupling efficiencies between two-dimensional (2-D) waveguides and microring resonators with a newly developed high-order discontinuous Galerkin time domain (DGTD) method for Maxwells equations. The DGTD method is based on a unified formulation for the physical media and the artificial media in the uniaxial perfectly matched layer (UPML) regions used to truncate the computational domain. The DGTD method employs finite element type meshes and uses piecewise high-order polynomials for spatial discretization of the Maxwells equations and Runge-Kutta methods for time integration. After demonstrating the high-order convergence of the DGTD method, the effect of separation gap between the waveguides and one and two microrings on the coupling efficiency and transmittance for pulse propagations is studied.

From Microscopic Theory to Macroscopic Theory: a Systematic Study on Modeling for Liquid Crystals

In this paper, we propose a systematic way of liquid crystal modeling to build connections between microscopic theory and macroscopic theory. In the first part, we propose a new Q-tensor model based on Onsager’s molecular theory for liquid crystals. The Oseen–Frank theory can be recovered from the derived Q-tensor theory by making a uniaxial assumption, and the coefficients in the Oseen–Frank model can be examined. In addition, the smectic-A phase can be characterized by the derived macroscopic model. In the second part, we derive a new dynamic Q-tensor model from Doi’s kinetic theory by the Bingham closure, which obeys the energy dissipation law. Moreover, the Ericksen–Leslie system can also be derived from new Q-tensor system by making an expansion near the local equilibrium.

WELL-POSEDNESS OF THE ERICKSEN-LESLIE SYSTEM

We prove the local well-posedness of the Ericksen–Leslie system, and the global well-posedness for small initial data under a physical constraint condition on the Leslie coefficients, which ensures that the energy of the system is dissipated. Instead of the Ginzburg–Landau approximation, we construct an approximate system with the dissipated energy based on a new formulation of the system.

Local Existence for the Dumbbell Model of Polymeric Fluids

Abstract A local existence and uniqueness theorem is proved for a micro-macro model for polymeric fluid, as well as the property of the solution. The polymer stress tensor is given by an integral which involves the solution of a diffusion equation, the coefficient of this diffusion equation depends on the gradient of the solution of the Navier–Stokes equation.

Boundary treatments in non-equilibrium Green's function (NEGF) methods for quantum transport in nano-MOSFETs

Non-equilibrium Greens function (NEGF) is a general method for modeling non-equilibrium quantum transport in open mesoscopic systems with many body scattering effects. In this paper, we present a unified treatment of quantum device boundaries in the framework of NEGF with both finite difference and finite element discretizations. Boundary treatments for both types of numerical methods, and the resulting self-energy @S for the NEGF formulism, representing the dissipative effects of device contacts on the transport, are derived using auxiliary Greens functions for the exterior of the quantum devices. Numerical results with both discretization schemes for an one-dimensional nano-device and a 29nm double gated MOSFET are provided to demonstrate the accuracy and flexibility of the proposed boundary treatments.